1. Field of the Invention
The present invention relates to capacitive accelerometers, and particularly to a differential capacitance inertial and gravity torque sensor using multiple voltage sources in order to compensate for inherent electrical asymmetries in the sensor, thus providing for a resonant mode of operation and enhanced sensitivity.
2. Description of the Related Art
Pivotal three-layer capacitive accelerometers are well known in the art, such as shown in U.S. Pat. No. 7,610,809, issued to McNeil et al. (the '809 patent). FIG. 1 and Column 1, line 36 to Column 2, line 41 of the '809 patent are hereby incorporated by reference. For example, FIG. 1 of the “809 patent shows an exploded side view of a conventional three-layer capacitive accelerometer constructed as a conventional hinged or “teeter-totter” type sensor. The capacitive accelerometer includes a pair of static substrates, which have opposed parallel planar faces. The substrates are spaced from one another and each has a plurality of metal electrode elements, of a predetermined configuration, deposited on one surface to form respective capacitor electrodes or “plates”. In an exemplary scenario, electrode elements operate as an excitation or sensing electrode to receive stimulating signals. The other electrode elements operate as the feedback electrodes for electrostatic rebalance. A single set of electrode elements operate as both sensing and feedback electrodes when the feedback signal is superimposed on the sensing signal.
In the capacitive accelerometer described in the '809 patent, a movable element, commonly referred to as a “proof mass”, is flexibly suspended between substrates by one or more rotational flexures situated at elevated attachment points for rotation about a rotational axis to form different sets of capacitors with electrodes. The movable element moves in response to acceleration, thus changing its position relative to the static sensing electrodes. This change in position results in a set of capacitors whose difference (i.e., a “differential capacitance”) is indicative of acceleration. Another set of capacitors for electrostatic rebalance is made up of the movable element and feedback electrodes. The feedback electrodes function to drive movable element to its reference position balanced between the sensing elements and maintain it there.
When in use as a teeter-totter type accelerometer the capacitive accelerometer described in relation to FIG. 1 of the '809 patent, a first section of the movable element, on one side of a rotational axis, is formed with relatively greater mass than a second section of the movable element, on the other side of the rotational axis. The greater mass of first section is typically created by offsetting the rotational axis such that an extended portion of the first section is formed distal from the rotational axis. In addition, the electrode elements are sized and spaced symmetrically with respect to the longitudinal axis L of the movable element. Similarly, the electrode elements are further sized and spaced symmetrically with respect to the rotational axis.
As noted in the '809 patent, two-layer and three-layer capacitive sensors having a teeter-totter configuration, however, tend to suffer from a number of drawbacks. In order to provide more capacitive output and, thus, better circuit performance (e.g., lower noise), the teeter-totter type capacitive accelerometer must have a relatively large proof mass. However, a large proof mass requires more die area, thus increasing cost and package size. Further, a proof mass should rotate as a rigid body, but the tendency for a proof mass to deform or bend increases in relation to its increasing size, particularly when it is subjected to high accelerations. This deformation or bending causes a non-linear effect that results in decreased accuracy of the sensor. For example, this nonlinearity can create direct current (DC) offset in the sensor output and possibly cause dysfunction of the system in which the accelerometer is deployed.
As further noted in the '809 patent, a particular problem of three-layer teeter-totter configurations, such as that illustrated in FIG. 1 therein, is that both the sensing electrodes and the feedback electrodes are clustered proximate the rotational axis. This configuration is inefficient in that the surface area of an extended portion, generally termed a “shield area”, of the movable element is unused. Further, the surface areas of the electrodes are relatively small due to their clustered configuration about the rotational axis. A smaller surface area of the sensing electrodes results in a lower capacitive output. A smaller surface area of the feedback electrodes provides insufficient actuation given voltage levels available from the feedback circuit.
Conventional capacitive sensors typically suffer from a number of drawbacks, including, for example, unwanted noise in the output signal. Capacitive accelerometers sense a change in electrical capacitance, with respect to acceleration, in order to vary the output of energized circuits. This energized circuit includes additional electronic gauges and high frequency generator, thus increasing susceptibility to additional electrical noise in the sensor output signals, limiting sensor sensitivity. Additionally, the sensitivity of a sensor, such as that shown in '809 patent, is limited because an elastic hanger of the proof mass (PM) has to be rigid enough to be used when this sensor is mounted on a vehicle or moving base, thus a PM's rotational natural frequencies about the rotational axis are on the order of about 12 Hz. Further, it is impossible to considerably change natural frequency oscillations of the PM in the '809 patent's sensor because of inherent electric asymmetry, which will be always present after sensor fabrication. Finally, even in principle, the sensor of the '809 patent cannot be used for measuring second derivative gravity potential because of a PM rotational axis offset between first and second ends, along with the inherent asymmetry of arrangement for sensing.
Thus, a differential capacitance torque sensor addressing the aforementioned problems is desired.